1. Field of the Invention.
This invention relates in general to a network communcations, and more particularly to a method and apparatus for efficient switching of partial minicells in ATM Adaptation Layer 2.
2. Description of Related Art.
ATM has been selected as a world standard for broadband ISDN in network communication systems. ATM systems have been implemented on a global basis and developed in a rapid growth. ATM technology is destined to play a major role in both public and private broadband networks. AAL2 is one of the four types of AAL (ATM Adaptive Layer) protocols which have been recommended by CCITT, namely AAL1, AAL2, AAL3/4 and AAL5. In general, the layer services provided by AAL1 are constant bit rate (CBR) services which require information to be transferred between source and destination at a constant bit rate. AAL2 offers a transfer of information with a variable bit rate. In addition, timing information is transferred between source and destination. Since the source is generating a variable bit rate, it is possible that cells are not completely filled and that filling level varies from cell to cell. AAL3/4 is used for transfer of data which is sensitive to loss, but not sensitive to delay. The AAL3/4 protocol may be used for connection oriented as well as for connectionless data communication. AAL3/4 itself does not perform all functions required by a connectionless service, since functions like routing and network addressing are performed on the network layer. AAL5 is designed to offer a service with less overhead and better error detection below the common part of the convergence sublayer (CPCS).
The AAL2 signaling protocol describes methods by which a switched AAL2 connection can be established between two AAL2 end users across a network that consists of both ATM and AAL2 switches. The current activities in the Study Group 11-WP1/Q6 or ITU-T is focused on specifying the requirements for such a signaling protocol. The important function of AAL2 signaling protocol is to establish an AAL2 connection between two AAL2 end points on a concatenation of ATM Virtual Channel Connections (VCCs) that are either on demand (SVC) or semi-pernanent (PVC). Some the basic requirements of AAL2 signaling protocol include the ability to establish an AAL2 connection between AAL2 end systems that support AESA formats, the ability to support hop-by-hop routing mechanism between AAL2 end systems, the ability to indicate any failures to corresponding management entity, and the ability to setup AAL2 connections with different QoS requirements.
The advantage of AAL2 to carry low bit rate and delay sensitive traffic on point-to-point links such as cellular trunking between base stations (BS or BTS) and mobile switching centers (MSC) has given impetus to AAL2 (minicell) switching. Minicell switching is being looked at seriously by many switch vendors since it offers seamless connectivity between mobile users, and mobile and wireline networks by avoiding intermediate transcoder conversion.
In an AAL2 switching network, peer to peer channel for each user is established through AAL2 signaling, and user packets (minicells) are switched like cells in an ATM network. The channel identifier (CID) mapping table, which, is an extension of the virtual path indicator/virtual channel indicator (VPI/VCI) table, is one of the many approaches that has been discussed for a possible solution to AAL2 switching.
At any given node, incoming CIDs on one VPI/VCI connection are mapped to outgoing CIDs on different VPI/VCI connections based on their destination, which was assigned during AAL2 signaling. The CID table approach is dynamic and efficient in terms of CID re-usability. The table 1100 in FIG. 11 illustrates one example of a CID table. In the table 1100 of FIG. 11, incoming VPIs 1102 are mapped to incoming VCIs 1104, incoming CIDs 1106, outgoing CIDs 1108, outgoing VPIs 1110, outgoing VCIs 1112 and the CID status 1114.
The main goal of AAL2 is to support low bit rate and delay sensitive applications in an ATM environment. AAL2 switching allows the low bit rate applications to share resources across an AAL switching network to improve the efficiency and avoid any protocol conversion. The key idea in minicell switching is that each intermediate node extracts the minicells embedded in an ATM cell and switches them in a different ATM cell belonging to the next hop in the chosen path.
During the cell assembly process, if the size of a minicell is larger than the remaining bytes of a partially filled ATM cell then the minicell is fragmented to fill the available bytes. The remaining portion of that minicell is assembled in the next ATM cell. Upon receiving an ATM cell, a node extracts the minicells embedded in it and if the node finds a partial minicell, it waits for the next ATM cell that contains the remaining part to arrive from the same source. The node performs a re-assembly on these two parts to recreate the complete minicell. The re-assembled minicell undergoes the AAL2 cell assembly in a new ATM cell that will be switched to the next hop.
The main drawback of this conventional approach is that each intermediate node along a selected path has to wait for the arrival of a complete minicell before switching any part of it to the next hop. The waiting time for a complete minicell to arrive at each intermediate node before switching introduces additional delay for delay sensitive traffic. In general, the end-to-end delay has an upper bound which is the sum of Timer-CU values, queuing delays at the previous nodes, reassembly delays (if PMS is not used), and propagation delays.
It is easy to see that the delay is proportional to the number of nodes in the selected path from source to the destination. The size of a data packet is much larger than the size of a voice packet requiring the intermediate node to have a large buffer to store all the information until the last byte is received from the source. Furthermore, each intermediate node is required to have the capability to perform re-assembly of the partial minicells, which is not done in current ATM networks.
It can be seen that there is a need for a more efficient scheme for switching minicells in an ATM AAL2 switching network.
It can also be seen that there is a need for a method and apparatus that allows partial cells to be switched without waiting for a complete minicell to be reassembled.
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a method for switching partial minicells in an AAL2 switching network without delaying to receive remainder partial minicell.
The present invention solves the above-described problems by providing an identification code directly on a remainder partial minicell to provide a means for routing partial minicells without having to reassemble the partial minicells into complete minicells.
A method in accordance with the principles of the present invention includes the steps of receiving a stream of ATM cells, wherein the ATM cells include complete minicells having an identification code, first partial minicells having an identification code, and remainder partial minicells having an identification code attached directly thereto, and transmitting each minicell immediately based upon the identification code when the minicell is a complete minicell or a first partial minicell and based upon the identification code attached directly to the minicell when the minicell is a remainder partial minicell.
Other embodiments of a system in accordance with the principles of the invention may include alternative or optional additional aspects. One such aspect of the present invention is that the step of transmitting further includes the steps of determining a size for a remaining payload for a current ATM cell, determining a size for the remainder partial minicell and inserting the remainder partial minicell in the current ATM cell when the size for the remaining payload is greater than the size for the remainder partial minicell.
Another aspect of the present invention is that the method further includes the step of determining whether a minicell is a complete minicell, a first partial minicell or a remainder partial minicell.
Another aspect of the present invention is that the step of determining further includes the step of comparing the identification code with entries in a identification code table.
Another aspect of the present invention is that a channel identifier table is provided that includes an incoming VPI field, an incoming VCI field, a incoming channel identifier code field; an outgoing channel identifier code field, an outgoing VPI field, an outgoing VCI field, a channel identifier code status field, a packet size field and a size received field.
These and various other advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and form a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to accompanying descriptive matter, in which there are illustrated and described specific examples of an apparatus in accordance with the invention.